Characterization of Magnesium Composites

Microstructure Evolution

Notably affecting corrosion resistance, the microstructure evolution of magnesium composites is the grain size, grain boundary, and phase distribution. Grain refinement leads to transform the density of grain boundaries and increases the mechanical properties as well as the corrosion resistance. Studies revealed the fraction of primary and secondary phases, and grain sizes are the key factors that control corrosion resistance. In metal matrix composites, the base matrix metal has a grain structure that can be refined and reformed to increase the properties of the composite material. Reinforcement is introduced in the metal matrix, which effects grain boundaries based on their shape and size. The size of the reinforcement is decreased, which will increase the grain refinement; whenever grain refinement is done the properties of the composite material increase. A sample with the smallest grain size and largest fraction of secondary phase improves the corrosion resistance because of the secondary phase, which affects galvanic corrosion and suppresses influence of grain size.

Mechanical Properties

Pure magnesium has very similar mechanical properties to bone properties. Later on, similar magnesium alloys were developed for the orthopedic applications. Magnesium alloys have three principal groups, the main group contains pure magnesium; the second group contains aluminum composites such as AZ91, AZ31, and rare earth elements like AE21;andthe third group contains WZ, WE, Mg-Ca, and MZ. The pure elements like Li, Ca. Al. Mn, Zn, Zr, and RE in magnesium combinations may extensively build the mechanical and physical properties of the composite by refining the grain structures, corrosion resistivity, increased strength, machinability, and formability because of the development of intermetallic phases.

Different contaminations usually found in the magnesium combinations are Be, Cu, Fe, and Ni, and a degree of impurities is allowed inside explicit points of confinement during the production of alloy. The plan of adequate levels for Be ranges from 2-4 ppm by weight, Cu is 100-300 ppm, Ni is 20-50 ppm, and Fe is 30-50 ppm. Both Ni and Be refrain from alloying components in biomedical applications considering their carcinogenic nature. The segments Mn, Ca, and Zn are major follow parts for the human body and RE segments showing antagonistic to hostile to cancer- causing properties should be the principal decision for assimilation into a combination. Melody et al. prescribed those especially little measures of uncommon earth components and other alloying metals; for instance, manganese and zinc present in human body condition may increase corrosion resistance. Mn is added to numerous composites to create corrosion resistance and decrease the destructive impacts of contaminations (Polmearet al., 1994).

New innovation has been happening in magnesium metallic materials. Magnesium composite materials have been developed and used as biocompatible, biodegradable implant material because of their nontoxic nature in the human body environment. Magnesium metal matrix material is reinforced with nutritional elements that may improve the mechanical properties as well as corrosion resistance. During the degradation process, the reinforcement acts as a filler material to the implanted tissue and heals the tissue without harm. If apatite-based reinforcements are used in the magnesium composite materials, the HAP or TCP could be increasing the mechanical properties and corrosion resistance, and helping to bond the fractured bone tissues into their original shapes.

It is very important to have enough and suitable mechanical properties for biomedical implants during their lives. Corrosive stability in the implanted structure is a high priority for patient safety because the role of an implant has to support physically damaged tissue throughout the healing process. Biodegradable implants have high mechanical properties and they have to represent best performance. Huge differences between the elastic moduli of implant material to damaged tissue may lead to elastic mismatches and cause stress shielding, especially in metallic biomaterials. Magnesium-based composites face two challenges: stress shielding effects in orthopedic implants and ductility limitation for cardiovascular stent applications. Many researchers have attempted to increase the mechanical properties of magnesium composites, which contain matrix alloys such as Al, Zn, Sr, Zr, Ca, and Mn. The alloy elements also offer the best mechanical properties as biomedical implant materials. The reinforcements are used to incorporate in the matrix, which increases the mechanical properties and corrosion resistance of the biomaterials. Calcium-based reinforcements mostly are used as reinforcements due to their excellent biocompatibility and degradation behavior in the physiological environment.

Biological Properties

Magnesium is one of the most important elements present in the human body where it occupies many number of enzymatic reactions. Magnesium as an element acts as protein, nucleic acid, and stabilizes plasma membranes of cellular activities. The average amount of magnesium present in the adult human body is around 2l-28g, and more than 50% of magnesium is presented in the bone tissue. Other soft tissues contain 35-40%, less than 1% is sequestered in the serum. Mg2+ ions play a vital role in formative bone frailty. Undeveloped bone tissues contain high concentrations of Mg2+ ions, but this concentration changes depending on age. The presence of magnesium in tissue increases elasticity of the bone. Magnesium-based composites are mostly used in orthopedic implants because they have significant effects on osteoblastic cell differentiation. Bone formation, and degradation of magnesium implants show its effect on accelerating on tissue healing. Magnesium presented in the biological environment begins to release hydrogen gas during the degradation process. The high amount of hydrogen gas release can cause tissue healing. The corrosion rate of magnesium should be restricted to decrease risk of gas obstacles during the healing process. By suppression of developed hydrogen gas in the physiological environment, Zn has a nature to control the gas evolution in the human body environment. If the Zn percentage is increased, it creates a toxic environment in the body. This has a wide range of mechanical properties, corrosive resistance, and also increases the osteoblastic immune system and enzymatic reactions. Magnesium composites may contain corrosive resistant, sufficient mechanical properties, and biocompatible and biodegradable matrix and reinforcement materials.

 
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